1. Field of the Invention
The present invention relates to a shading correction circuit, and more specifically to a shading correction circuit for performing shading correction in digital camera signal processing and a digital camera signal processing circuit which employs the shading correction circuit.
2. Description of the Related Art
Conventionally, shading correction is used only in order to keep the signal level uniform. For example, a circuit which includes a shading level correction circuit and a noise removal circuit independent of each other is disclosed in Japanese Patent Laid-Open No. 003569/1988.
FIG. 7 shows an example of a conventional shading correction circuit. Referring to FIG. 7, the shading correction circuit shown has an input terminal 34 and an output terminal 40 and includes a multiplication type AD converter 35, a noise removal circuit 42 including a sample hold circuit 36 and a low-pass filter 37, a shading correction coefficient storing memory 38, a CPU 41, and an address generator 39. The address generator 39 produces an address for outputting a correction coefficient corresponding to a pixel. The address is connected to an address input of the shading correction coefficient storing memory 38, and the shading correction coefficient storing memory 38 outputs a correction coefficient corresponding to a pixel. The correction coefficient is multiplied by an image signal inputted from the input terminal 34 by the multiplication type AD converter 35. A result of the multiplication is inputted to the sample hold circuit 36 and further inputted to the low-pass filter 37, by which noise of the multiplication result is removed. When a shading correction coefficient is produced, a predetermined value is inputted in advance to the shading correction coefficient storing memory 38, and a signal obtained by picking up an image of uniform surface lighting is inputted from the input terminal 34. Then, the inputted signal is multiplied by a constant by the multiplication type AD converter 35, and noise removal of the signal is performed by the sample hold circuit 36 and the low-pass filter 37. Then, a correction coefficient with which a signal to be inputted to the CPU 41 may be constant is calculated by the CPU 41 and is written into the shading correction coefficient storing memory 38.
However, the conventional shading correction circuit has the following problems.
In shading correction, while one screen is multiplied by different coefficients, since this is simple multiplication, the SN ratio is maintained and the noise level rises. When such simple noise removal means is utilized as in the conventional shading correction circuit, if a filter which can remove amplified noise is applied, then also a band of a video signal is removed. Even if a circuit wherein an edge component is removed by slice processing and a resulting signal which includes only a noise component is subtracted from the original signal is adopted in order to prevent occurrence of such a trouble as just described, since the noise level differs among different portions of an image after the shading correction, if a simple level slice is used, then a noise component and an edge component cannot be separated from each other.
Accordingly, since, in level correction of shading, different coefficients are used for multiplication of pixels at different portions of an image, also the noise level differs among different portions of the image. Therefore, there is a problem that, if a simple noise removal circuit is utilized, then an edge becomes dull and much noise remains at different portions of an image.